Envelope tracking (ET) is a known technique for improving the efficiency of power amplifiers. In a conventional implementation of an envelope tracking technique, a voltage signal at the drain input of a radio frequency (RF) power amplifier (PA) is varied to be proportional to the envelope of a RF signal. Tracking is performed in order to match the dynamic range of the supply voltage of the power amplifier to the instantaneous requirements of the RF signal envelope.
A subclass of the ET technique is partial envelope tracking (PET). A conventional PET circuit replaces the tracking below a certain envelope voltage level by providing a constant voltage of the power supply to the drain input of the RF PA. A conventional PET circuit also tracks the envelope peaks above that voltage level. One of the advantages of a PET technique is low power consumption and, specifically, low power consumption of a power supply when tracking signals with strongly varying envelopes. Examples for such signals include long-term evolution (LTE) signals, wireless local area network (WLAN) signals, and the like.
Advanced wireless and handled devices designed for LTE and WLAN communications are based on multi-input-multi-output (MIMO) architectures. In such architectures, the information (input) signal is distributed and transmitted via several transmit channels.
As illustrated in FIG. 1, each transmit channel includes a power amplifier 110 connected to an antenna 120. The antennas 120-1 through 120-M form the MIMO architecture. In order to allow the partial envelope tracking of transmitted RF signals, each power amplifier 110 is connected to a PET circuity that includes a voltage enhancement circuitry (VEC) 140 and an excess envelope calculator (EEC) circuit 130. For example, a power amplifier 110-1 is connected to a VEC 140-1 and VEC 140-1 is connected to EEC 130-1.
Generally, each EEC 130, e.g. EEC 130-1, determines the mode of operation of the respective VEC 140, e.g. VEC 140-1, based on an access of the input signal (el). The operation may be either a tracking mode or a normal mode. The tracking mode is active when the envelope of the transmitted signal e1 is above a predefined threshold. In this mode, the voltage signal VD provided to the power amplifier is continuously adapted according to the changes of the envelope of the transmitted signal in order to match the required dynamic range of the power amplifier. In the normal mode, the voltage signal VS is provided to the power amplifier from the power source 150.
Referring to FIG. 2 where the operation of the PET circuity including the VEC 140 is shown, it should be noted that the description provided with reference to FIG. 2 is applicable for each VEC 140 shown in FIG. 1. In a conventional implementation, the VEC 140 includes a main valve (MV) 142 connected to the power amplifier (RF AMP) 110 and the power source 150 that outputs a voltage signal (VS) filtered by a capacitor (CS). The VEC 140 further includes an envelope tracking function realized by a tracking unit 145 that includes at least a tracking valve (TV) 145-1 connected to a feedback resistor (RFB) and a linear feedback amplifier (FB AMP) 145-2.
The VEC 140 also includes a diversion valve (DV) 147 which is connected in series to a grounding valve (GV) 148. In a conventional implementation, a Voltage Control Unit (VCU) 141 activates normal mode when the envelope ‘e’ of a transmitted RF signal is below a predefined voltage threshold. The tracking mode is active when the envelope ‘e’ of the transmitted signal is above the predefined voltage threshold. In this mode, the voltage signal provided to the power amplifier is continuously adapted according to the changes of the envelope of the transmitted signal in order to match the required dynamic range of the power amplifier.
Also connected in the VEC 140 is a storage capacitor CT 144, which, together with DV 147, allows a smooth transition between the normal and tracking mode. Specifically, during the normal mode, the storage capacitor 144 is charged at the voltage level provided by the power source and during the tracking mode the storage capacitor 144 is discharged. When the VEC 140 switches from the normal mode to the tracking mode, a source of a drain current to the power amplifier 110 switches from the power source 150 to a current path through the DV 147 and the storage capacitor 144. To minimize voltage fluctuations of the drain voltage VD provided to the power amplifier, the capacitance of the storage capacitor CT 144 is large, typically several microfarads. Therefore, the size of the capacitor in terms of area is also large.
Consequently, whether the storage capacitor is implemented as a discrete component or as part of an integrated circuit (IC), the size of the storage capacitor 144 is relatively big. This problem is magnified when more than one storage capacitor is required in a system that includes more than one transmit channel (e.g., the system shown in FIG. 1). For example, for a MIMO-based system with 4 transmit channels, 4 storage capacitors are required.
As conventional PET circuits are large in size and primarily designed to support power amplifiers that are stand-alone modules, such circuits cannot provide efficient solutions for MIMO-based systems. In particular, such conventional PET circuits cannot be efficiently utilized in handheld wireless devices, for example, smartphones and tablet computers in which the size is a critical constraint.
It would therefore be advantageous to provide a PET solution that would overcome the deficiencies noted above and be efficiently implemented in MIMO-based wireless handheld devices.